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They have assigned some Memory area for MODBUS RS 485 serial communicating to third party DCS. In that there is one bit M525.0 which is not used in logic(Checked with Cross reference options), but DCS generated alarm 2 times which is mapped to this bit M525.0

So to test the mapping in DCS I have forced this bit in VAR table and I have witnessed the alarm in DCS.

Is it possible to check whether it is used with Indirect addressing ?

Also there is less chance of using this bit in Indirect addressing because the next bit M525.1 and M525.3 used in logic with Direct addressing.

If you used both methods and still come up blank you have inherited a lucky program that surely does some indirect addressing. You'd better start checking any of the FC or FB which are called in your CFCs for your modbus for ANY pointers and block moves using code 16#83 for memory area. Hopefully you have the source for those blocks.